The term “wireless communications network” is used herein to refer to a communications network comprising at least one base station transceiver arranged to communicate with at least one mobile station. It will be appreciated by those skilled in the art that a wireless communications network will often take the form of a cellular communications network, in which a plurality of base station transceivers each define a geographical cell. Mobile stations located in the communications network communicate with one or more base station transceivers, for example, the closest one to the mobile station. Each base station transceiver has a limited range and a cell can be considered to be a geographical region over which a base station transceiver can communicate effectively with a mobile station therein.
Mobile stations such as mobile telephones may be located within a cellular communications network to send and receive signals to and from the base station transceivers. Each mobile station operating within a cell requires a certain amount of bandwidth to operate and because the total bandwidth of base station transceivers is limited the number of mobile stations which can operate within a cell is limited.
The provision of base stations is expensive. Firstly, the location and surveying of suitable sites for base stations is time consuming and complex since the location of any one base station impacts the base station requirements for adjacent cells. Furthermore, obtaining planning or zoning permission for base stations is becoming increasingly difficult as a result not least of concerns about electromagnetic emissions and the aesthetic impact of antenna towers.
Accordingly, there is a general desire to minimise the number of base stations. This may be achieved by improving the coverage of base stations i.e. the geographical area over which sufficient radiated powers are produced to allow effective communication with mobile stations and/or increases in capacity i.e. the number of mobile stations which may be supported by a single base station. Assuming that these aims may be met without a disproportionate increase in base station costs, it is generally understood that a reduction in the number of base stations is desirable.
One traditional approach to this problem has been to increase “sectorisation” at the base, i.e. to use a single base station location to provide coverage in different “sectors” which are arranged radially around the base station location.
Many existing systems use “tri-sectoring” in which three sectors are covered using a single base station. The prior art tri-sectoring arrangement increases both uplink and downlink capacity by a factor of almost three compared to a basic omni-directional arrangement.
However, the improvement is less than three times since perfect sectorisation is not possible i.e. there is always some overlap between adjacent sectors. This loss is called partition loss and typically increases with the number of sectors at the base. Using multibeam technology to replace each sector with an array, allows beams to be formed which are narrower than the full sector width. This has a similar effect to increased sectorisation and thus improves capacity. These arrays may for example comprise a plurality of columns of antenna elements which may or may not be combined using a beam former to produce a lesser or equal number of beams by combining the outputs of the columns, for example using a Butler matrix. For N columns there can be up to (and including) N orthogonal beams.
Typically each such sector uses an antenna having a plurality of elements which provide a plurality of beams. Typically three fixed beams are used both for uplink and downlink connections in each sector. Thus in the prior art, an antenna may be used to provide three sector coverage with three beams in each of these sectors. Using an array for each sector, narrower beams may be used which may be directed when the antenna is configured, to different parts of the sector. Since the beams typically overlap, additional information is available on the uplink which may be used using conventional space-time signal processing techniques, to provide additional spatial processing gain in the uplink.
These steps have gone some way towards providing increased capacity in base stations. Nevertheless, it is anticipated that capacity requirements will increase three or four fold from present day levels within a short space of time. This capacity requirement cannot be met with a three or four fold increase in the number of base station sites for at least the reasons explained above. Thus yet further improvements to base station capacities are required.
U.S. Pat. No. 6,480,524 describes a six column array for the downlink using a three way beam former which gives good capacity benefits. Whilst it would be possible to produce three beams from a three column array, the use of a six column array allows more control of the beam shape. By shaping the beams with “deep cusps” (by using multiple elements for each beam) overlap between the beams is reduced which in turn reduces “partition loss”. This partition loss is characterised in part by interference between adjacent beams directed to different mobile stations within a sector and also by increased hand-off overhead as mobile stations hand-off back and forth between overlapped beams. Thus in the case of the downlink, there are significant advantages in reducing overlap, as explained in U.S. Pat. No. 6,480,524.
In terms of apparatus size, for the downlink it would be possible to use a six column array at the top of the mast, and maximum downlink capacity would be achieved by forming six beams with this array. However, each downlink beam would require an expensive power amplifier, together with cabling up the mast. As mentioned above, using fewer beams than columns also provides improved beam shapes. Thus, a good cost/capacity tradeoff for the downlink is a 6 column array with 3 deep cusp beams.
A further consideration in base station design is the possibility of using the same antenna array both for the uplink and downlink. The use of separate arrays requires larger areas of land and also generally has increased aesthetic impact. Where the arrays are mounted on the same structure, additional arrays also produce increased wind loading problems. Thus it is generally desirable to attempt to use the same array i.e. a common array, both for the uplink and downlink communications. However, this may place compromises on the design of the uplink antenna which typically is configured (i.e. constructed and fed) identically to that of the downlink.
Thus in practice, the capacity of a state-of-the-art base station is asymmetric i.e. greater downlink capacity is available than uplink capacity. Whilst this may be suitable for some data applications such as web-browsing or streaming video, it is unsuitable for applications such as voice communications.